1. Field of the Invention
The invention relates to a method for operating an internal combustion engine having a combustion method with homogeneous auto ignition.
2. The Prior Art
Combustion methods are generally known in which a homogeneously pre-mixed fuel/air mixture is ignited by auto ignition. These combustion methods are frequently referred to as HCCI (homogeneous charge compression ignition). In order to achieve homogeneous combustion, a sufficiently high energy level must be present in the cylinder. This can be brought about, for example, by a high compression pressure and/or by a sufficient amount of hot residual gas. Control of the parameters that influence auto ignition, such as compression, exhaust gas reflux rate, injection time, fuel distribution, air filling, etc., is very complex. For example, a method for controlling the combustion sequence, in which an HCCI combustion process is modeled by internal status variables, and the output variables of the modeled combustion process are used to regulate the setting variables that influence the auto ignition is described in German Patent No. DE 102 37 328 B4. In this connection, setting variables can be the control of inlet and outlet valves of the internal combustion engine, for example, which can be variably controlled with regard to their stroke and their on time. Both the compression and the exhaust gas reflux rate, in the case of internal exhaust gas reflux, can be influenced by variable on times. Additional influence parameters can be the setting of an exhaust gas reflux valve, an adjustable compression ratio, or the injection parameters. In this connection, the operating mode of homogeneous auto ignition cannot be represented in all operating points, but rather are mostly limited to the range of partial load. It is difficult to manage the process, particularly in the marginal ranges and at dynamic operating transitions. Because of the leanness in the case of homogeneous auto ignition, the amount of torque that can be given off is limited. Furthermore, the characteristic field range is limited in the direction of a lower speed of rotation and load, in that it has to be possible for auto ignition to even take place. Up to the present, the combustion method is used practically only in limited characteristic field ranges, at almost stationary operation. In broad operating ranges, the internal combustion engine is operated in conventional manner.
Control systems are previously known for conventional combustion methods, which calculate a torque reference value for the torque to be given off by the internal combustion engine from the driver's demand (e.g. gas pedal position), on the basis of the torque. An example of such a structure is German Patent No. DE 196 18 849 A1. Proceeding from a desired torque determined by the driver's demand, a corrected desired torque is calculated by working-point-specific values read out of characteristic fields, for the ignition angle degree of effectiveness and the lambda degree of effectiveness, which torque is subsequently converted into a reference value for filling, and set by the throttle valve.